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Abstract

A fast narrowband transmittance model, referred to as the Fast Fitting Transmittance Model (FFTM), is developed based on rigorous line-by-line (LBL) calculations. Specifically, monochromatic transmittances are first computed from a LBL model in a spectral region from 1 to 25000 cm-1 for various pressures and temperatures ranging from 0.05 hPa to 1100 hPa and from 200 K to 320 K, respectively. Subsequently, the monochromatic transmittances are averaged over a spectral interval of 1 cm-1 to obtain narrowband transmittances that are then fitted to various values of absorber amount. A database of fitting coefficients is then created that can be used to compute narrowband transmittances for an arbitrary atmospheric profile. To apply the FFTM to an inhomogeneous atmosphere, the Curtis-Godson (C-G) approximation is employed to obtain the weighted effective coefficients. The present method is validated against the LBLRTM and also compared with the high-spectral-resolution measurements acquired by the Atmospheric Infrared Sounder (AIRS) and High-resolution Interferometer Sounder (HIS). With a spectral resolution of 1 cm-1 and a wide spectral coverage, the FFTM offers a unique combination of numerical efficiency and considerable accuracy for computing moderate- to high-spectral-resolution transmittances involved in radiative transfer simulations and remote sensing applications.

Comparison of the mean spectral transmittances for water vapor, which are computed from the LBLRTM algorithm and the present fitting method for an inhomogeneous path from 100 to 0 km. The mid-latitude summer atmospheric profile is used.